In motor graders, the rear wheels are usually driven directly by the engine through a transmission and differential gearing. Further, it is known to supplement the main drive by means of a hydrostatic front wheel drive system which includes a fluid pump driven by the engine for providing pressurized fluid to fluid motors. The motors in turn drive the front wheels thereby supplementing the main rear wheel drive. In the past, supplemental hydrostatic drives have been developed which automatically shifted between two or more torque levels in response to the transmission ratio and/or hydraulic system pressure. These systems were continuously powered to provide supplementary hydrostatic drive for the main drive but had no provision for operation only on demand when the main drive loses traction. Moreover, since the supplemental hydrostatic drive is notably less efficient than the main direct drive, it is desirable to reduce unnecessary utilization of the hydrostatic drive.
In some motor graders manufactured by the assignee herein, the vehicle operator is able to manually control the torque produced by the supplemental drive. These vehicles include an open loop control wherein the pump and motor displacement in the supplemental drive are adjusted in response to a manually operable control lever. However, such a system does not provide maximum efficiency since control of the hydrostatic drive is at the operator's discretion. Therefore, it is desirable to provide a system in which operation of the hydrostatic system is automatically controlled to maintain maximum operating efficiency.
One system which attempts to overcome the problems associated prior systems is disclosed in U.S. Pat. No. 4,186,816 which issued to Pfundstein on Feb. 5, 1980, hereinafter referred to as '816. The '816 patent discloses a closed loop electronic speed feedback system for automatically controlling the supplemental drive system of a motor grader. More specifically, closed loop feedback electronics control a servo actuated pump that is connected by a hydraulic system to a pair of front wheel hydrostatic drive motors. The supplemental drive for the motor grader has three modes of operation. The first mode is the "off" mode in which the front drive wheels are free running and unpowered. The second mode is the "normal" mode where the control system allows a predetermined amount speed differential between the main and supplemental drive wheels before the supplemental hydrostatically driven wheels begin to supplement the main drive wheels. The third mode is an "overspeed" mode where the control system provides a predetermined percentage of overspeed of the auxiliary drive wheels to provide a continuous, positive supplementary drive.
As mentioned above, hydrostatic drives are much less efficient than direct drives and, therefore it is desirable to reduce unnecessary utilization of the supplemental hydrostatic drive whenever possible. To facilitate more efficient power utilization, it is desirable to provide a system wherein the responsiveness of the supplemental drive decreases with increasing ground speed. This is because wheel slip in the main drive is more likely to occur at lower vehicle speeds. The '816 system completely disables the supplemental system in higher gear ratios of the main transmission. However, when the supplemental drive is enabled a given speed difference between the front and rear wheels will result in the same level of supplemental assist from the front wheels irrespective of ground speed.
Furthermore, when the '816 system operates in the "aggressive" mode, the supplemental drive system is continuously operated regardless of the degree of slip between the rear wheels and the ground. Under good tractive conditions, such as on dry hard surfaces, the "aggressive" mode should not be used because continued utilization of the supplemental drive is inefficient and undesirable. Instead it is desirable to provide a more responsive system wherein the level of supplemental drive assist is controlled in response to the degree of slip between the rear wheels and ground.
The present invention is directed towards addressing one or more of the problems set forth above.